Heating of a metal strip in a heat-treating furnace has been performed mainly by indirect heating using a radiant tube. The indirect heating restricts productivity because, in addition to large thermal inertia, valid heat input into a metal strip becomes difficult as the difference between the temperature of metal strip and furnace temperature becomes small. Furthermore, in the indirect heating using a radiant tube, for example, for a steel sheet such as a carbon steel, it is difficult to perform rapid heating near the transformation point at which endothermic reaction occurs, and to perform high-temperature annealing because of restriction by heat resistance of the radiant tube, restricting choice of degrees of freedom of heat treatment conditions for metal strip.
In contrast, induction heating in which a metal strip is heated with high-frequency current is capable of freely controlling heating speed and heating temperature, so that the induction heating has large degrees of freedom at the points of heat treatment operation and development of metal strip products and is a heating method that has been paid attention recently.
The induction heating is largely categorized into two methods. One is an LF (longitudinal flux heating) system for heating a metal strip by flowing high-frequency current in an induction coil surrounding the circumference of a metal strip to make magnetic flux pass through a cross section in the longitudinal direction (traveling direction) of the metal strip to generate induction current circulating in a cross section in the width direction of the metal strip perpendicular to the magnetic flux.
The other method is a TF (transverse flux heating) system for heating a metal strip by arranging inductors (sufficient magnetic bodies) around which respective primary coils are wound to sandwich the metal strip and flowing currents in the primary coils to make the magnetic fluxes generated by the currents pass through sheet surfaces of the metal strip via the inductors to generate induction currents in the sheet surfaces of the metal strip.
In the induction heating by the LF system for making induction current circulate in a sheet cross section, on the basis of the relationship between current penetration depth δ and current frequency f(δ(mm)=5.03×105√(ρ/μr·f), ρ(Ωm): specific resistance, μr: relative magnetic permeability, f: frequency (Hz)), when the current penetration depth of the induction current generated on the front and back of the metal strip is deeper than the thickness of the steel sheet, the generated currents interfere with each other, generating no induction current in a cross section of the metal strip.
For example, in the case of a non-magnetic metal strip or a steel sheet that loses magnetic properties over its Curie temperature, current penetration depth δ becomes deep, generating no induction current when the sheet thickness of the metal strip is thin. Furthermore, even in the case of magnetic material, no induction current generates in a cross section of steel sheet in the LF system when the sheet thickness is too thin as compared with penetration depth.
On the other hand, in the induction heating by the TF system, magnetic flux passes through a sheet surface of the metal strip, enabling the metal strip to be heated regardless of sheet thickness and difference of magnetism and non-magnetism, but heating efficiency is lowered or heating is entirely impossible in some cases when opposite inductors are not adjacent. Furthermore, overheating readily occurs at ends of the metal strip disadvantageously (for example, see Japanese Patent Application Laid-Open (JP-A) No. S63-119188).
Furthermore, when a magnetic metal strip is not located at the center of opposing inductors, the magnetic metal strip may be pulled to one of the inductors to make magnetic flux be concentrated regionally to increase temperature variation of the metal strip. Furthermore, in the induction heating by normal TF system, the inductor is difficult to be easily changed in its shape, disadvantageously making it difficult to cope with change in sheet width of the metal strip.
For that reason, for example, an electromagnetic induction heating device has been disclosed in Japanese Patent Application Laid-Open (JP-A) No. S59-205183 that includes a magnetic pole segments arranged in parallel with a sheet width direction of a sheet to oppose the sheet and independently movable in the thick direction of the sheet and a movable masking shield made of a non-magnetic metal capable of appearing and retreating in the sheet width direction of the sheet for adjusting magnetic field generated by the magnetic pole segments.
The electromagnetic induction heating device of JP-A No. S59-205183 is capable of adjusting magnetic flux in response to change in sheet width of the sheet, but is difficult to rapidly adjust magnetic flux in sheet width direction when sheet width of the sheet is largely changed.
Japanese Patent Application Laid-Open (JP-A) No. 2002-008838 discloses an induction heating device having a plurality of independent magnetic bars and equipped with a magnetic circuit with a variable width adaptable to the width of a metal strip. However, in the induction heating device of JP-A No. JP 2002-008838, an example is illustrated in which magnetic cores movable in a width direction is provided near respective induction coils placed apart from front and back sides.